Polarized beams of electromagnetic radiation can be singly, partially, or multiply polarized and can be characterized by representative characteristic parameters such as:
degree(s) of temporal coherence, PA0 degree(s) of spatial coherence, PA0 wavelength bandwidth content, PA0 degree(s) of collimation determining angular bandwidth, and intensity. PA0 1. Rotatable Element or Intensity Modulating Rotating Element Ellipsometers (REE); and PA0 2. Phase Modulating Modulation Element Ellipsometers (MEE). PA0 Stationary Polarizer(s); PA0 Stationary Compensator(s); PA0 Stationary Analyzer(s); PA0 Rotatable Polarizer(s); PA0 Rotatable Compensator(s); PA0 Rotatable Analyzer(s); PA0 Rotating Polarizer(s); PA0 Rotating Compensator(s); PA0 Rotating Analyzer(s); and PA0 Modulator Element(s). PA0 a. Rotatable Element Nulling Ellipsometers (RENE); PA0 b. Rotatable Element Automated Nulling PA0 Ellipsometers (REANE); PA0 c. Modulation Element Ellipsometers (MEE); PA0 d. Rotating Analyzer Ellipsometers (RAE); PA0 e. Rotating Polarizer Ellipsometers (RPE); PA0 f. Rotating Compensator Ellipsometers (RCE); PA0 g. Rotating Polarizer and Analyzer Ellipsometers (RPAE); PA0 h. Rotating Polarizer and Analyzer, Fixed Compensator (RPAFCE); PA0 i. Rotating Analyzer and Compensator, Fixed Polarizer Ellipsometer (RACFPE); PA0 j. Rotating Polarizer and Compensator, Fixed Analyzer (RPCFAE); PA0 k. Rotating Analyzer, Fixed Polarizer and Compensator Ellipsometer (RAFPCE); PA0 l. Rotating Polarizer, Fixed Analyzer and Compensator Ellipsometer (RPFACE); PA0 m. Rotating Compensator, Fixed Analyzer and Polarizer Ellipsometer (RCFAPE);
Continuing, in ellipsometer and polarimeter settings, expected experimentally obtained intensity values of polarized beams of electromagnetic radiation can be calculated using mathematical models which assume primarily "coherent", primarily "incoherent" and mixtures of "coherent" and "incoherent" addition of electric field components present therein. Coherent addition of electric fields to arrive at an Intensity is accomplished by: EQU (I=(E1+E2+. . . En)*(E1+E2+. . . +En)*),
and incoherent addition of electric fields to arrive at an Intensity is accomplished by: EQU (I=(E1*E1*)+(E2*E2*)+. . . +(En*En*)).
(It is noted that each of the En's in the incoherent electric field addition Intensity equation can result from coherent addition of electric fields which result, for instance, from a multiplicity of electromagnetic waves which reflect from a single region on a patterned sample system).
To appreciate the present invention, it is to be understood that to date most work in ellipsometry has been focused upon investigation of relatively homogeneous regions in a sample system, and mathematical modeling of investigated sample systems and electromagnetic radiation utilized in investigation thereof, has proceeded with the assumption that coherent addition of electric fields accounts for experimentally measured intensities. That is, with a few exceptions, (see supra herein), the occurance of incoherent addition of electric fields in a beam of electromagentic radiation as a cause of intensity thereof has been largely unreported.
Incoherent addition of electric fields, and simultaneous coherent and incoherent electric field addition in electromagnetic radiation to produce a measurable intensity, however, can result where a polarized electromagnetic beam is caused to, for instance, interact with a sample system with identifiably separate laterally offset pattern regions present therein. This is the case as a portion of such a beam of electromagnetic radiation which travels one spatial pathway can have a definite phase relationship to portions of said beam of electromagnetic radiation which travels an alternative spatial pathway, thereby entering coherence related interference effects into production of an intensity signal and requiring that intensity be calculated by (I=(E1+E2+. . . En)*(E1+E2+. . . +En)*), but it can also occur that portions of said beam of electromagnetic radiation which travel one spatial pathway have a lack of a distinct phase relationship to portions of said beam of electromagnetic radiation which travel an alternative spatial pathway, such that phase relationships between them are essentially negligible and require that intensity be calculated by (I=(E1*E1*)+(E2*E2*)+. . . +(En*En*)), which describes incoherent addition of electric fields.
Continuing, ellipsometer, polarimeter and the like systems, allow determination of sample system physical, (such as thickness), and/or optical properties, (such as refractive index and extinction coefficient of one or more surface films therein or thereon). In particular, ellipsometer systems detect change in "Polarization State" of a beam of polarized light which is caused to interact with a Sample System, where Polarization State here refers to a set of values for Polarized Light Beam Orthogonal Components, (such as "S" and "P"), Magnitude Ratio, and a Phase Angle therebetween. (For general introductory background purposes, it is noted that "P" refers to that electric field component of a beam of electromagnetic radiation which is in a plane containing the normal to an investigated Sample System surface and an incident and/or transmitted beam of polarized light, and "S" refers to an electric field component of said beam of electromagnetic radiation which is perpendicular to said "P" component, and simultaneously parallel to the surface of said Sample System. It is also noted that a "full" polarization state also requires designation of an absolute value to which a magnitude ratio is referenced, and the direction of rotation of electric fields associated with a polarized beam of light).
Conventional use of ellipsometer systems provides the ability to analyze a sample system, and via appropriate transfer equations (specific to an ellipsometer system type), or other mathematical technique, (eg. regression), as applied to experimentally determined intensity data, return PSI and DELTA values which are representative of a "spot" on a Sample System, which "spot" presents with relatively constant vertically oriented thickness(es) and optical properties. Said PSI and DELTA are related as: EQU r.sub.p r.sub.s =Tan(.psi.)e.sup.i(.DELTA.)
where r.sub.p and r.sub.s are the well known Fresnel Reflection Coefficients for P and S polarized electromagnetic radiation. (Note similar Transmission Fresnel Coefficients, t.sub.p and t.sub.s, can be substituted to provide a similar equation for the case where a beam of electromagnetic radiation is caused to be transmitted through a Sample System. A book by Azzam and Bashara titled "Ellipsometry and Polarized Light", North-Holland, 1977 further describes Fresnel Coefficients, and is incorporated into this Disclosure by Reference).
Conventional teachings are then directed to use of ellipsometer systems to determine physical and/or optical properties of sample systems which are essentially homogeneous over a laterally dimensioned region "spot" of a sample system, upon which a beam of polarized electromagnetic radiation impinges, (that is the "spot" investigated on the sample system is essentially determined by an electromagentic beam spot size). Ellipsometry has not been widely applied in characterizing sample systems which contain patterns, (eg. sample systems which present with a plurality of identifiably separate laterally disposed regions present within a beam of polarized electromagnetic radiation "beam spot" size, at the location where a beam of electromagnetic radiation impinges upon a sample system). Of importance in the present invention, is the fact that where "patterned" samples are investigated by a beam of electromagnetic radiation, effects appear which do not appear in investigation of homogeneous sample systems. Said effects, in addition to mixed coherent and incoherent addition of electric fields, can also include shadowing, (where a beam of electromagnetic radiation caused to impinge thereupon and an angle to the surface thereof does not have access to certain regions on said patterned sample system adjacent to relatively vertically taller regions thereon).
A recent paper titled "Multiwavelength Ellipsometry for Real-Time Process Control of the Plasma Etching of Patterned Samples", by Maynard et al., J. Vac. Sci. Technol., B 15(1), Jan/Feb 1997, describes real-time determination of thickness of a film as it is etched. The technique involves analysis of ellipsometer detector provided intensity based upon an assumption of addition of coherent electromagnetic beam portions which simultaneously interact with different laterally disposed regions on a patterned sample system. The mathematical model reported in this paper, however, does not consider that electric fields in electromagnetic radiation which is caused to interact with a patterned sample system can combine incoherently to produce detector provided intensity instead of, or in addition to, coherent addition of electric fields, and assumes that the effects of diffraction are negligible.
A paper titled "In Situ Ellipsometry and Reflectometry During Etching Of Patterned Surfaces: Experiments And Simulations", by Haverlag and Oehrlien, J. Vac. Sci. Technol. B 10(6), Nov/Dec 1992, describes the effect of pattern orientation on a sample system investigated by ellipsometry. It is concluded that cases wherein a probe beam is directed parallel to, and cases wherein a probe beam is directed perpendicular to sample system line patterns must be modeled differently, but that etching procedure end point detection is possible. For the case where the probe beam is oriented parallel to sample system pattern lines, a sudden change in PSI-DELTA plane plots is observed at an etch procedure end point. Such a sudden change in PSI-DELTA plane plots is not observed at an etch procedure end point where the probe beam is oriented perpendicular to sample system pattern lines, however, and it is concluded that etch procedure end points in low vertical height to lateral dimension aspect ratio sample system patterning, in such a case, can be very difficult to detect. End point detection by the monitoring of a derivative of reflectivity is identified as an alternative technique.
A paper by Blayo et al, titled "Ultraviolet-Visible Ellipsometry For Process Control During The Etching Of Submicron Features", J. Opt. Soc. Am. A, Vol. 12, No. 3, March 1995 concludes that there exists an optimum spectral range that can be used for process control for etching patterned wafers. That is, said article identifies the importance of observing PSI and DELTA values at an appropriate wavelength in a spectroscopic range, (eg. 375.8 nm rather than 632.8 nm where etching of a specified multilayer stack was investigated), as sensitivity to the results of an etch process reported in this work do not appear in PSI and DELTA plots obtained at most wavelengths.
A paper tiled "In Situ And Ex Situ Applications Of Spectroscopic Ellipsometry", Mat. Research, Soc. Proc., Vol. 324, 1994, by Woollam et al., describes application of ellipsometry to investigation of single films, flat panel displays, and in situ semiconductor growth and deposition control. Tracking of variables such as PSI, DELTA and refractive indicies as functions of independent variables such as wavelength and sample system layer depth, is identified.
Another paper by Maynard and Hershkowitz, titled "Thin-Film Interferometry Of Patterned Films", J. Vac. Sci. Technol. B, Vol 13, May/June 1995, describes the use of interferometry to determine the thickness of thin films on patterned sample systems. The mathematical modeling and analysis techniques are applicable to sample system pattern features which are smaller than the wavelength of light utilized to investigate them. This is essentially equivalent to saying that coherent addition of electric fields, with any accompanying interference effects, is assumed.
A paper titled "Optical Etch-Rate Monitoring: Computer Simulation Of Reflectance", by Heimann et al., J. Electochem. Soc., Vol. 131, No. 4, April 1984, reports numerical simulation of a signal from a laser etch-rate monitor which utilizes changes in reflectance from a multilayer structure while the top layer is being etched. Another paper titled "Optical Etch-Rate Using Active Device Areas: Lateral Interference Effects", by Heimann, J. Electochem. Soc., Vol. 132, No. 8, August 1985, describes the analysis of Reflectance data to determine end point etching of polysilicon.
A paper titled "Spectral Ellipsometry On Patterned Wafers", by Duncan et al., SPIE, Vol. 2637, April 1995 describes application of a modulation element ellipsometer in investigation of a one-dimensional etched, rectangular groove geometry pattern on a semiconductor substrate. Zeroth-order reflection coefficients for orthogonal P and S polarization states are utilized as are models for ellipsometric PSI and DELTA. The effect of groove geometry on PSI and DELTA is investigated.
A paper titled "Optical Analysis Of Complex Multilayer Structures Using Multiple Data Types", by Johs et al., SPIE, Vol. 2253, 1994, describes the importance in considering back-side reflections in multiple layer sample systems. A paper by Jellison titled "Sample Depolarization Effects From Thin Films of ZnS on GaAs As Measured By Spectroscopic Ellipsometry", Appl. Phys. Lett. 61 (5) Aug. 1992 is also identifed as providing insight of results investigation of thin films with thickness gradients present therein. Another paper which is expected to be published in Thin Film Solids" in 1997, was presented by Jellison at the Second International Conference on Spectroscopic Ellipsometry in Charleston, N.C. Reported were the results of investigation of uneven depth film utilizing incoherent addition of electric fields, but not discussed was investigation of patterned sample systems wherein separately identifiable laterally disposed regions are present thereon, which sample system geometry, it is noted, can require mathematical modeling which assumes partially coherent and partially incoherent addition of electric fields in arriving at an electromagnetic beam intensity value which is measured at a detector for a singly, multiply or partially depolarized beam of electromagnetic radiation which is caused to interact with a patterned sample system and enter said detector.
Continuing, ellipsometer and polarimeter systems can be generally, broadly classified as:
An example of a rotating analyzer ellipsometer (RAE) system, for instance, is presented in a Patent to Woollam et al., U.S. Pat. No. 5,373,359. Additional Patents to Johs et al. and Green et al., U.S. Pat. Nos. 5,504,582 and 5,521,706 respectively provide further insight into rotating analyzer ellipsometer (RAE) systems. Another U.S. Pat. No. 5,416,588 to Ducharme et al., describes a Modulation Element Ellipsometer (MEE). While the specifics of signal generation are different in (REE) and (MEE) ellipsometers, and even amongst Ellipsometers of similar type, the end result of utilization thereof is provision of PSI and DELTA values , (or similar related parameters such as Fourier Coefficients or Mueller Matrix related parameters "N" "IS" and "C" identified by Jellison for modulation element ellipsometer systems), for Sample Systems analyzed therein. This is the case regardless of Sample System type (eg. isotropic, anisotropic, or anisotropic and depolarizing).
Numerous other Ellipsometer Systems could be described, which are, for instance, comprised of various combinations of:
Examples of Ellipsometers which can practice the method of the present invention method are, for instance:
(Note that similar identifying descriptions also apply to Polarimeter and the like Systems and that for the purposes of the present invention it is not necessary to describe each above listed system in detail.)
Regardless of ellipsometer type, however, a common result is the production of an intensity signal by a detector system which is positioned to intercept an electromagnetic beam which has interacted with a sample system, which intensity signal is the result of all effects caused by ellipsometer system components and a sample system interacted with. As well, it is beneficial to consider that the effects of all components of a specific ellipsometer system can be eliminated from collected data, leaving only information present which pertains to an investigated sample system per se., (ie. only the effects of the interaction of a polarized beam of electromagnetic radiation with a sample system, on said polarized beam of electromagentic radiation, remain). This concept should be kept in mind regarding the present invention because, again for emphasis, the present invention can be practiced with essentially any ellipsometer/polarimeter system wherein a change of polarization state in a beam of singly, partially or multiply polarized electromagnetic radiation, which change results from the interaction thereof with a sample system, is analyzed to provide insight to optical and physical properties of said investigated sample system.
As alluded to, until recently, (eg. Maynard et al. and other papers), published results of the application of ellipsometry have focused upon the investigation of sample systems which are essentially non-depolarizing and where an electromagnetic beam spot caused to impinge upon a sample system falls entirely on a region of said sample system which does not contain patterned edges or regions. Ellipsometry has not been generally adapted to simultaneously investigate a plurality of identifiably separate laterally disposed regions (with respect to one another), on a patterned sample system.
A need for improved and expanded ellipsometry procedures which provide for electric fields to be modeled as being incoherently combined to allow calculation of experimentally determined detector provided intensity instead of, or in addition to, coherent addition of electric fields, has thus been identified. It is disclosed that the J. A. Woollam Co. WVASE, (Registered Trademark), computer program, with enabling adaptation, is applicable to facilitating practice of the present invention method.